Out of curiosity and as a ConTeXt stress test, I installed 2014.07.07
MkIV and recompiled my book with it, and looked at the pixel-by-pixel
comparison between it and the version compiled with 2014.05.17.

There were many changes.  I am going through them one at a time in case
any could be bugs in luatex or context.  (I noticed several good changes
where pages fit more precisely into their height.)

Here is a change that might be a bug.  It uses the following minimal
example:

\setupbodyfont[palatino]
\showframe
\setuplayout[width=5.4286in]

\starttext

Let's see whether this estimate is reasonable.  According to the
plane's technical documentation, the 747-400's four engines together
can provide roughly 1~meganewton of thrust.

\stoptext

With 2014.05.17, the linebreaks are as follows, with no overfull boxes:

  Let's see whether this estimate is reasonable. According to the plane's
  technical documentation, the 747-400's four engines together can provide
  roughly 1 meganewton of thrust.

With 2014.07.07, the linebreaks are as follows, but with an overfull box:

  Let's see whether this estimate is reasonable. According to the plane's tech-
  nical documentation, the 747-400's four engines together can provide roughly
  1 meganewton of thrust.  

The "roughly" on the second line protrudes a good way into the margin.
From the log file:

  Overfull \hbox (8.80678pt too wide) in paragraph at lines 7--10
  \3>palatino-12pt-rm-tf-0--0 ni-cal doc-u-men-ta-tion, the 747-400’s four en-gines to-gether can pro-vide roughly

Both runs use luatex 0.79.1 (svn rev 5033), which I compiled by hand and
then did 'context --make' to regenerate the formats.

Is this change in linebreaking intended, or could it be an unintended
consequence of another change (and therefore maybe problematic)?

Perhaps related, the PDF files behave differently with copying and
pasting.  Copying the paragraph from the 2014.05.17 file, I get spaces
between the words.  Copying from the 2014.07.07 file, I get no spaces
between the words (using mupdf).  (The difference may be due to the
smaller interword spaces, whose size may have fallen below a threshold
that mupdf uses.)

I've attached the uncompressed PDFs (each about 14K).

-Sanjoy